1. Field of the Invention
The present invention relates to a coherent combining type demodulator in a communication system and method thereof, and more particularly, to an apparatus for coherent combining type demodulator and method thereof, by which signals received by a receiver are demodulated wherein the signals were spectrum-spread by orthogonal code to be transmitted from a transmitter via multi-paths.
2. Discussion of the Related Art
FIG. 1 is a block diagram of a transmitter by IS-95A standards used for CDMA (code division multiple access) communication system.
Referring to FIG. 1, a transmitter consists of an encoder 10, an orthogonal modulator 20, a spreader 30, base band filters 40a and 40b, mixers 50a and 50b, and an adder 60.
The encoder 10 encodes inputted information bits according to a predetermined coding type. The orthogonal modulator 20 modulates the encoded bits based on the orthogonal quadrature modulation method. The spreader 30 spreads the orthogonal-modulated signals by PN codes of in-phase component (hereinafter abbreviated ‘I component’) and PN codes of quadrature component (hereinafter abbreviated ‘Q component’) to output spread signals of I and Q components, respectively. The base band filters 40a and 40b perform waveform shaping to the spread signals of I and Q components. The mixers 50a and 50b apply carrier frequencies to the outputs of the I and Q components from the base band filters 40a and 40b to modulate, respectively. And, the adder 60 adds outputs of the mixers 50a and 50b to each other to output a signal s(t).
A signal transmitted from a transmitter is affected by fading due to multi-paths during transmission, whereby a receiver receives a combined signal of symbols transmitted along different paths. In order to demodulate such a combined signal according to the multi-paths efficiently, CDMA communication system adopts a rake receiver.
Generally, a rake receiver includes a multitude of fingers to demodulate a combined signal according to multi-paths.
FIG. 2 is a block diagram of a rake receiver including a plurality of fingers 1a to 1n according to a related art, and FIG. 3 is a detailed block diagram of one of the fingers in FIG. 2.
An operation of the related art receiver is explained by referring to FIG. 2 and FIG. 3 as follows.
An orthogonal quadrature despreader (hereinafter abbreviated ‘despreader’) 19, as shown in FIG. 3, despreads reception signals by applying PN codes of I and Q components to the reception signals rx_i and rx_q of the I and Q components, respectively. First and second Hadamard transformers 15a and 15b find Walsh correlation values for 64 Walsh codes, e.g., Hadamard codes, for I and Q components from the signals despread by the despreader 19, respectively. An energy detector 16 accumulates the Walsh correlation values of the I and Q components for the 64 Walsh codes, respectively, and adds the I and Q components of the accumulated Walsh correlation values to output 64 Walsh energy values.
Referring to FIG. 2, an adder 4 of a combiner 2 performs maximum-ratio combination on the corresponding energy values for the 64 Walsh codes of the fingers 1a to 1n to generate 64 outputs, and a dual maxima 5 finally outputs six symbols for the 64 Walsh codes (one Walsh code set). A decoder 3 decodes the outputs of the combiner 2 to output information bits. In the above-explained method, the combiner 2 adds the energy of each of the fingers 1a to 1n. So, it is called a rake receiver according to a non-coherent combining method.
Performance of a method of demodulating and decoding reception signals based on the above-mentioned non-coherent combining method is degraded by the loss generated from a non-coherent combining process, thereby being inferior to that of a method of demodulating and decoding reception signals based on the coherent combining method. For instance, when output signals of two fingers having the same S/N are combined, the coherent combining method enables to provide a combining gain of 3 dB in general. Yet, the non-coherent combining method has a non-coherent combining loss of about 1 dB, thereby providing a combining gain of 2 dB.